The regulation to exhaust gases of vehicle is spread worldwide. Exhaust gases contain hydrocarbons (HC's), carbon monoxide (CO), and nitrogen oxides (NOx), for purification of which is employed a ternary powdery catalyst in which a porous carrier, such as alumina (Al2O3), carries particles of (a) noble metal(s), such as platinum (Pt), palladium (Pd), and rhodium (Rh).
Noble metals provide a catalysis, which promotes associated reactions as a contact reaction on an exposed surface of noble metal. For a noble metal particle, the activity of catalysis is thus proportional to the surface area. To provide a small amount of noble metal particles with a maximized catalytic activity, such particles should be prepared possibly small in size, to be high of specific surface area, and evenly dispersed over a carrier, with their small sizes maintained.
Noble metal particles prepared small in size exhibit high activities in surface reaction due to great surface energy, and very unstable, so that they tend to agglomerate when brought close. In particular, Pt particles have a remarkable tendency to sinter when heated, and even if dispersed on a carrier, they can be sintered to be large in size, with reduced catalytic activities. For vehicle-oriented catalysts, which are exposed to hot temperatures, typically within 800° C. to 900° C. and sometimes over 1000° C., it is difficult to hold particle size of noble metals prepared.
To control agglomeration of noble metal particles, one may reduce the surface energy. However, for such reduction to be effective, noble metal particles should be so great as 50 nm or 100 nm in size. Such a great size of noble metal particle fails to provide a sufficient catalytic activity.
An alternate measure is the use of an anchor for anchoring a noble metal particle on a carrier.
Japanese Patent Application Laid-Open Publication No. 59-230639 has disclosed a catalyzer in which a honeycomb substrate carries an activated alumina, which has held thereon, for carrying, particles of at least one of cerium (Ce), zirconium (Zr), iron (Fe), and nickel (Ni), at least one of neodymium (Nd), lanthanum (La), and praseodymium (Pr), as necessary, and at least one of Pt. Pd and Rh.